Unlocking the Secrets: How Walnut Trees Reveal Clues About Human Sex Determination

The genetic basis for the changing sexes of walnut trees was elucidated by researchers at the University of California, Davis. The study, published in Science, uncovers a mechanism that has remained stable in walnuts and their ancestors for the past 40 million years—and exhibits similarities to sex determination processes in humans and other animals.

Flowering plants employ diverse strategies to prevent self-pollination. Some achieve this by designing flowers that complicate self-pollination; certain species possess distinct “male” and “female” plants. Others time-separate their male and female flowering phases. Trees related to walnuts, hickories, and pecans take this concept a step further.

A walnut tree generates flowers of one sex, followed by the other within the same season, yet individual trees vary in which type blooms first. Some trees consistently exhibit “male-first” or “female-first” flowering patterns, a phenomenon noted by Charles Darwin in 1877. In the 1980s, a graduate student at UC Davis, Scott Gleeson, discovered that this characteristic was governed by a single genetic locus.

“Walnuts and pecans display a temporal dimorphism, alternating male and female flowering throughout the season,” stated Jeff Groh, a graduate student in population biology at UC Davis and the principal author of the paper. “This has been recognized since the 1800s but was not understood at the molecular level until now.”

This occurs in both cultivated walnuts and their wild relatives, such as the Northern California black walnut. In wild varieties, the occurrence of male-first and female-first trees is nearly equal.

Groh, along with his doctoral mentor, Professor Graham Coop from the Department of Evolution and Ecology, utilized data from UC Davis’s walnut breeding initiatives and monitored flowering patterns in native Northern California black walnut trees surrounding the UC Davis campus. By categorizing them into male-first or female-first groups, the researchers sequenced their genomes and pinpointed sequences related to this trait.

Identical mechanism, distinct genes

In walnuts, two variations of a gene are associated with female-first or male-first flowering. This genetic polymorphism is present in at least nine species of walnut and has remained stable for nearly 40 million years.

“It’s quite unusual to preserve variation over such an extended period,” remarked Groh. In this scenario, the two flowering variations counterbalance each other. If one flowering type gains prevalence in the population, the less frequent type acquires a reproductive edge, leading it to become more widespread. This drives the system toward a 50:50 balance and sustains genetic diversity.

Pecans, Groh discovered, possess a similarly balanced genetic polymorphism determining flowering order, though located in a different region of the genome than that of walnuts. The pecan polymorphism appears to be more ancient than that in walnuts, dating back over 50 million years.

How did walnuts and pecans, being closely related, develop an identical flowering mechanism through notably different genes?

It may be that the ancestors of walnuts and pecans encountered similar evolutionary solutions as they adapted over time. However, it is also plausible that this temporally separated flowering system originated much earlier in this family, around 70 million years ago, but the specific genetic mechanisms used to achieve it have transformed over time.

Interestingly, this phenomenon bears similarity to the operation of animal sex chromosomes, which maintain equilibrium with two structural variants (X and Y chromosomes in humans and other mammals).

“There is a clear analogy to a common method of sex determination,” Groh concluded.